If either realpart or imagpart is a float, the non-float is converted to a float before the complex is created. If imagpart is not supplied, the imaginary part is a zero of the same type as realpart; i.e., (coerce 0 (type-of realpart)) is effectively used.

Type upgrading implies a movement upwards in the type hierarchy lattice. In the case of complexes, the type-specifier must be a subtype of (upgraded-complex-part-type type-specifier). If type-specifier1 is a subtype of type-specifier2, then (upgraded-complex-element-type 'type-specifier1) must also be a subtype of (upgraded-complex-element-type 'type-specifier2). Two disjoint types can be upgraded into the same thing.